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Resolution of the human sex hormone-binding globulin dimer interface and evidence for two steroid-binding sites per homodimer

Item Type:Article
Title:Resolution of the human sex hormone-binding globulin dimer interface and evidence for two steroid-binding sites per homodimer
Creators Name:Avvakumov, G.V. and Grishkovskaya, I. and Muller, Y.A. and Hammond, G.L.
Abstract:Human sex hormone-binding globulin (SHBG) transports sex steroids in the blood. It functions as a homodimer, but there is little information about the topography of its dimerization domain, and its steroid binding stoichiometry is controversial. The prevailing assumption is that each homodimeric SHBG molecule contains a single steroid-binding site at the dimer interface. However, crystallographic analysis of the amino-terminal laminin G-like domain of human SHBG has shown that the dimerization and steroid-binding sites are distinct and that both monomers within a homodimeric complex are capable of binding steroid. To validate our crystallographic model of the SHBG homodimer, we have used site-directed mutagenesis to create SHBG variants in which single amino acid substitutions (V89E and L122E) were introduced to produce steric clashes at critical positions within the proposed dimerization domain. The resulting dimerization-deficient SHBG variants contain a steroid-binding site with an affinity and specificity indistinguishable from wild-type SHBG. Moreover, when equalized in terms of their monomeric subunit content, dimerization-deficient and wild-type SHBGs have essentially identical steroid binding capacities. These data indicate that both subunits of the SHBG homodimer bind steroid and that measurements of the molar concentration of SHBG homodimer in serum samples have been overestimated by 2-fold.
Keywords:Binding Sites, Dimerization, Sex Hormone-Binding Globulin, Steroids
Source:Journal of Biological Chemistry
ISSN:0021-9258
Publisher:American Society for Biochemistry and Molecular Biology (U.S.A.)
Volume:276
Number:37
Page Range:34453-34457
Date:1 January 2001
Official Publication:https://doi.org/10.1074/jbc.M106274200
PubMed:View item in PubMed

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